Just put in what is recommended and if its not showing up in the window, get a buddy to center the bike and gently move the bike left to right while you keep looking in the window. You should see the level move by at some point.

Remeber too, if your checking with a dipstick method, to wait a few minutes for all the oil to drain down from the engine head before checking the level. If you dont you could run the risk of overfilling.